So I'll keep it short. When you install your new modem there is a call to the ISP (Spectrum) to enable it. They used to only ask for the MAC address but be prepared for more questions.
Most folk use what was installed and add their own router for what reasons they needed the new router for. Some will just try it without configuration of the ISP gear, some will set the first cable/modem router to bridge. Since bridge mode is widely discussed I stop here.
For a long time I use to get internet from my ISP modem and I used my own WiFi router to get WiFi to devices. But most recently I got Spectrum service with cable and the ISP installed and Arris router that seems to be a modem and wireless router because I had never seen a wireless router with a coaxial cable. I was so used to seeing the coax in a modem and then Ethernet from it to a wireless router. Now, the question is if I'm able to remove the Arris and place my own modem and plug and Ethernet from it to a wireless router. Or is there a special set up I have to do or permission from ISP